The invention disclosed herein relates generally to semiconductor pressure sensing apparatus, and more particularly to such apparatus employing a piezoresistive stress sensitive element mounted in a low cost housing employing premolded elastomeric seals.
It is well known to package piezoresistive stress sensitive elements so that they are adapted to sense fluid pressure. In order to obtain an output indicative of fluid pressure, such a stress sensitive element must be interfaced with other structure in at least two respects which may have significant effects on the output of the element. Specifically, the element must be mechanically supported, and a fluid tight joint must be provided between the element and the support structure to enable fluid pressures to produce a force differential between different portions (typically opposite sides) of the element.
The quality requirements for the fluid tight joint may vary depending on the intended application of the pressure sensing assembly. A hermetic seal is generally required for high pressures or sensing pressures from low capacity static sources. Hermetic sealing can normally be achieved only with a hard seal, such as produced by soldering, welding thermocompression bonding, electrostatic bonding, etc.
For lower pressure applications involving constantly changing pressures or high capacity static sources, forming the joint with an adhesive or soft seal may be suitable. In any event, care must be taken to minimize stresses applied to the stress sensitive element by the seal and/or any mounting structure. Stresses in the stress sensitive element may be created by differences in the thermal coefficients of expansion of the element and any structure bonded thereto. Such stresses may also result from aging and contraction of adhesives and/or the force required to grip the stress sensitive element between seals.
A further disadvantage associated with adhesives is that they must be applied in an uncured form in which they are relatively fluid. Consequently, it is difficult to assure close control over the cured adhesive configuration, as is necessary to achieve satisfactory and consistent performance from piezoresistive transducers. The small size of the semiconductor chips and other transducer parts (typically tenths of an inch or less) increases the difficulty in achieving adequately precise control over the adhesive. Further, depending on the geometry of the parts, the uncured adhesive may tend to flow into depressions and cavities in which it is unwanted. Finally, the most common adhesives which exhibit satisfactory elasticity and other required characteristics when cured are not compatible with some fluids whose pressures must be sensed.
It is pointed out that because of basic differences in the mechanism by which pressure is sensed in piezoresistive stress sensitive elements and in other sensors, such as capacitive pressure cells, mounting and pressure seal techniques suitable for capacitive and other types of transducers may be difficult to satisfactorily implement in piezoresistive transducers. A reason for this is that in a capacitive transducer, the region of maximum sensitivity is at the center of a diaphragm structure. Therefore, a capacitive pressure cell may be readily gripped or supported and/or a pressure seal provided at the periphery of the cell without producing an unacceptable effect on the transducer output signal.
Conversely, a piezoresistive stress sensitive element is quite sensitive to forces applied at the periphery of its diaphragm. Therefore, the materials used for mounting means and pressure seals must be carefully chosen and the mounting means and seals carefully designed to avoid unacceptable effects on the output signal.
An apparent solution is to design the piezoresistive stress sensitive element such that the active diaphragm area with the piezoresistive device thereon is relatively small in diameter, and that the mounting and pressure seal is located at a substantially larger distance from the center of the diaphragm. Such an approach has several problems, including a requirement for a larger semiconductor chip which is expensive, increased physical size of the completed transducer when miniaturization may be preferred or required, and/or reduced sensitivity due to the small active diaphragm area.
A further consideration in the design of a piezoresistive pressure transducer involves insuring that the transducer is compatible with fluids whose pressures are to be sensed, and providing that the sensor design is compatible with as wide a range of fluids as possible. With reference to the semiconductor chip, the most critical area is that at which external electrical connections are made with the doped circuit elements. It is known to use a variety of conformal coatings to protect the electrical connections, as well as other chip features. For silicon substrates such coatings include varnishes, dimethyl silicone, silicon dioxide and silicon nitride.
Coatings such as varnishes are subject to aging effects and can affect transducer sensitivity and repeatability. Dimethyl silicone can also affect transducer sensitivity, repeatability, and response. With regard to silicon dioxide and silicon nitride, in order to facilitate the application, the coating is normally formed over the entire silicon surface rather than just the interface between the doped circuit elements and external conductors. As noted in U.S. Pat. No. 3,417,361 issued to H. Heller et al on Dec. 17, 1968, and discussed in detail in U.S. Pat. No. 4,125,820 issued to J. Marshall on Nov. 14, 1978, most metals are at least somewhat soluble in silicon, and silicon dioxide or silicon nitride will, during its formation, take up some kinds of dopants from the semiconductor substrate. This may alter the dopant concentration in the piezoresistors and/or other doped elements, and affect transducer performance. In addition, most usual coating materials, including silicon dioxide and silicon nitride, have significantly different thermal coefficients of expansion than silicon. The use of such coatings may result in thermal stresses which, in turn, will be reflected in the transducer output signal.
A further important consideration in pressure transducer design is an increasing demand for general cost reductions and for low cost transducers which meet moderate performance requirements. Thus, it has become increasingly important to devise piezoresistive pressure transducer designs which are adaptable for various modes of operation and usable with a variety of fluid media. Finally, low cost generally implies a design which utilizes low cost materials and is adaptable to automated assembly.
The applicant has devised a unique piezoresistive pressure transducer design utilizing a low cost housing and premolded elastomeric seals, which design reduces the need for protective coatings by locating the piezoresistive element-external circuitry interface outside of the area contacted by the fluid whose pressure is being sensed, and which is well suited for automated assembly.